There are probably wholesalers who sell journals just for that purpose. As far as your craft store retailers opinions- I think it will just depend on their commitment to the idea of handcrafted work. Is their work high end and completely hand crafted? Is some of it cheaper and more saleable?
Whichever method you choose, be completely honest about how it's made and what is made by you. Play up the handmade paper if you choose to use mass produced journals.

I love handmade books and personally wouldn't buy a journal just for the handmade paper cover. I like the special bindings and papers. But you're right- one can only afford so many of them.

Have you tried teaching workshops on book making? Sometimes it's easier to make money teaching and selling the supplies, than it is to sell the finished product.
I paid $30 to take a 2 hour class and I'm paying $50 to take a 2 day workshop in April.
I'm a metalsmith, but I like the idea of bookbinding, so I take short workshops when I can.

I don't know about the legality, although I agree with leafylady that somewhere journals for this purpose probably already exist.

I have recently started bookmaking myself and while it is taking me some time to do each project, it doesn't take so long as to make a similar project that I might see in a store too expensive. And, it will get easier as I get more skilled. The part that takes the longest for me is choosing the materials (and finding time in my overextended days to work on the projects). Since you make your own papers, the selection stage must be easier for you. I guess I'm saying that the handmade book/journal seems to be a unique category. I'd be willing to pay good money for a beautiful and well-made one.

I've seen various sorts of kits out there for bookbinding, which include pre-cut boards and even holes for screw and post binding. Perhaps online you could find even more varieties of pre-cut boards in different sizes and for different types of binding??

I have done the journals made for covering and the albums with the post bindings, but none of the paper included is creative.

The problem I found in making from scratch was the amount of handmade paper. For a whole book to be handmade it takes at least a week to make all the paper and another to allow it dry then I have to finish it so that the ink won't bleed, another week. And one more to put it together.

But I think last night in bed I decide I wouldn't be comfortable with something only half-mine.